A smog check repair station will first have to find out why your Ford Ranger
is running rich, and more then likey producing high CO. The diagnosis for
this fault should begin with inspection of your Ranger's fuel injection
"feedback" system. This includes ensuring the oxygen sensors are working
properly, your Ford's emissions computer is receiving the signals from the
oxygen sensor, the computer is computing the data properly and sending the
correct signals to the fuel injectors to either increase or decrease fuel
delivery to the combustion chambers.
The feed back test will utilize a 5-gas analyzer (usually a smog machine)
and propane. The smog technician should introduce propane to your Ford
Ranger's intake system and ensure the oxygen sensors are reading this
introduction as an increase in CO, and ordering the computer to decrease
fuel delivery at the instant propane is added to the system. This test
should be done after the engine has sufficiently warmed up and while it is
running at idle. If no change in fuel delivery is seen we know we have a
feedback problem. Then the diagnose should turn to which part of the
feedback system is defective. Using a voltmeter the technician should
observe the voltage output of the oxygen sensor as propane is added. Voltage
should increase. If there is no increase we have a defective O2 sensor. The
sensor should be replaced and the test started over. If voltage increase is
present at the sensor output the technician needs to ensure voltage increase
is also present at the computer input terminal. If voltage is present at the
terminal as well next the technician must ensure the computer output
terminal to the fuel injectors are lowering "injector pulse rates".
At the end of the feedback test it will be know whether the rich fuel fault
is electronic or mechanical, and the technician can further diagnose the
fault.
